For example, Patent Literature 1 discloses a so-called single-link piston-crank mechanism in which reciprocating motion of a piston is converted into rotational motion of a crankshaft by a con-rod. The con-rod includes a small end portion connected with a piston pin, a large end portion connected with a crankpin of a crankshaft, and a rod portion which combines the small end portion with the large end portion.
In the technique of Patent Literature 1, a bearing metal interposed between the large end portion and the crankpin includes a tubular radial bearing portion which is slidably in contact with the crankpin. For purpose of improving a surface contact between the bearing metal and the large end portion, a back surface of the radial bearing portion which is joined to the large end portion of the con-rod is constituted by a straight cylindrical surface and curved cylindrical surfaces. The straight cylindrical surface extends concentrically with its rotation axis whereas the curved cylindrical surfaces are curved in a barrel shape continuously from front and rear end portions of the straight cylindrical surface.
However, the technique of Patent Literature 1 is premised on the so-called single-link piston-crank mechanism, but does not deal with a possibility of the multi-link piston-crank mechanism. Moreover, the large end portion of the con-rod in the single-link piston-crank mechanism does not necessarily have the same structure as a linking member rotatably attached to the crankpin in the multi-link piston-crank mechanism.
That is, Patent Literature 1 merely examines a shape which improves the surface contact of the bearing metal slidably abutting on the crankpin, in the field of single-link piston-crank mechanism.
In the field of multi-link piston-crank mechanism, an improvement on the surface contact of the bearing metal slidably abutting on the crankpin remains room of examination.